Kiln operation



Dec. 23, 1969 G. CREMER ET'AL 3,485,900

KILN OPERATION Filed Feb. 5, 1968 4 4 Sheets-Shet 1 INVENTORS Dec. 23,1969 G. CREMER ET AL 3,485,900

KILN OPERATION Filed Feb. 5, 1968 4 Sheets-Sheet 4.

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F \F I IR H A 1 1 INVENTORS am ne/0 a e vie AKA/5 HS United StatesPatent Int. Cl. F27b 9/ 24; F27d 3/12 U.S. Cl. 263-52 14 Claims ABSTRACTOF THE DISCLOSURE A tunnel kiln in which oppositely placed burnersproduce streams of combustion gases making a loop-shaped pattern, andthe opposite burners are alternately turned gradually off and on so thatthe loop pattern is moved transversely across the interior of the kiln.

The present invention relates to the heating of kilns, particularlythose of the tunnel variety, used for the firing of ceramics, forexample.

One important problem to which the present invention is addressed isthat of achieving and maintaining a substantially even temperature in akiln or furnace heated by burner means using gaseous or liquid fuel.Until the present time, this problem has not been satisfactorily solved.

Pottery or ceramic kilns typically have very large volumes often on theorder of several hundred cubic yards. The required firing temperature insuch kilns may for example easily be as much as 1,250" C., with apermitted departure of only 1 /2 segar cones. The allowable temperaturevariation at any given part of the kiln or kiln cross-section thus isabout i C., a requirement which is very difiicult to meet in actualpractice. With kilns having substantially smaller firing spaces, forinstance in the case of trolley-type kilns having a volume less than 10cubic yards, it is very diflicult to insure that the temperature isuniform throughout the firing space.

In the case of conventional burners using gaseous or liquid fuels, thereis a relatively small zone of heat localization and build-up immediatelyin front of the burner nozzle, and this represents a substantial factortending to prevent the achievement of even and uniform temperaturethroughout the firing space. In the past such burners have been arrangedin burner chambers or recesses located in the Walls so as to maximizetheir spacing from the articles to be fired, in hopes that anequilibrium could be achieved between the high temperature at the burnernozzle and the gases formed by combustion in the burner space.Unfortunately, these measures have failed to solve the underlyingproblem.

Thus, other attempts to solve this problem have been made. The so-calledjet or beam burners which have in the past developed are an example ofother approaches to the solution of the problem. These burners operatewith a comparatively high pressure at the nozzle, both of the gas andalso of the combustion air, so that there is a high speed injection ofair and gas into the kiln. Owing to this high speed there is a verypronounced eddy formation in the atmosphere inside the firing space. sothat there is less variation in temperature throughout the firingchamber. However, in the case of kilns with large firing spaces, the useof these burners does not achieve the required substantially eventemperature. This is because the high blow-in or injection speeds of thegases from the burners produces an annular eddy in the firing space withcomparatively still or quiescent conditions in the center of the firingchamber, i.e., the move- 3,485,900 Patented Dec. 23, 1969 ment of thekiln atmosphere is restricted to a circular peripheral one. It isapparent that such a flow system in a kiln atmosphere will lead to verynoticeable temperature differences.

In practice, use is usually made of oppositely-placed burners directedtowards one another. In such a case there are two relatively quiescentelongated heat zones running along the axis of the tunnel kiln, and thegaseous heating products move around in an annular eddy. Although thismanner of operation leads to an improvement over operation with only asingle jet burner, the result obtained is definitely not completelysatisfactory.

One object of the present invention is to provide a kiln and method ofoperating a kiln which in the simplest possible manner brings about apractically complete solution of the problem of achieving andmaintaining a substantially even or uniform temperature in all parts ofthe firing chamber of a kiln.

The invention consists in a method of heating a firing chamber or spacethrough the use of gaseous or liquid fuels by using at least two burnerswhich are arranged opposite each other and directed towards one another,characterized in that the quantity of the fuel-air mixture introducedand/ or the heating effect or output of the two oppositely placedburners is varied periodically. As a result there is no establishment ofstationary directions of movement of the heating gases from the burners,and instead the paths along which the heating gases move are themselvesin continuous movement. In accordance with a further development of theinvention, it may in many cases be advantageous to make the cyclicalperiods of variation of the burners of different lengths, so that thereis a steadily changing rhythm in the movement of the heating gases.

If gas or oil burners with varying rates of heat output are used, it isquite possible to obtain the aforementioned variation by varying thequantity of fuel with-out altering the quantity of air, providing thatthe variations in heat output do not exceed certain limits. This is trueeven though from the stoichiometric point of view it would naturally bedesirable to vary the amount of combustion air in accordance withvariation in the amount of fuel; however, actual practice often demandsthat operation be carried out with an excess of air, so that even if thesupply of fuel is varied, complete combustion will take place.

In accordance with the method or process of the invention, twooppositely-disposed burners are always present. Since the sum of thequantities of fuel injected into both burners remains constant at everymoment, a sufiicietly complete combustion at each burner, despitevariations in the quantity of fuel injected, is achieved if the amountof air blown in does not undergo any alteration, since the momentaryexcess of air present at one burner will be available for combustion atthe opposite burner.

The simplest case from the theoretical point of view is that in whichthe quantity of air of each burner is varied in accordance with the rateof fuel supply. In this case the rate of injection of material by theburners will correspond with the rate of heat developed.

In accordance with a further and a preferred feature of the invention,the cycles of variation of the two 0ppositely placed burners are of thesame length, and are offset by in phase. Thus, if the rate of injectionand/ or heat developed of one burner is reduced, that of the otherburner is correspondingly increased. As a result, the eddies formed movein a distinct rhythm between the two burners, with a horizontalreciprocatory or shuttling movement.

In accordance with a further feature of the invention, the sum of thequantities of fuel-air mixture injected and/ or of the heat outputs ofthe two oppositely placed burners directed towards one another will atevery instant correspond with the quantity of air and fuel and/ or heatrequired by that part of the kiln to be heated by the two burners.

In accordance with a further development of the invention, the pairs ofburners arranged along the axis of a tunnel kiln are regulated so thatthey vary their fuel and/ or heat output with a difference in phase ofup to 180 between adjacent sequentially-placed burners. As a result, thereciprocatory movement of the kiln atmosphere has superimposed on it awave-like movement moving along the kiln.

A kiln for carrying out the method of the invention for the firing orheat treatment of discrete articles, for instance for the firing ofceramics, preferably comprises oppositely placed burners which aredirected towards one another at a level above the surface on which thegoods to be fired are stacked, the goods being stacked in such a manneras to leave alleys or channels at the same level as the burner fuel andair nozzles, so that the currents from the burners come towards eachother along the alleys or channels.

A trolley-type tunnel kiln for carrying out the method in accordancewith the invention advantageously has the further feature that below thesurface on which the goods are stacked, alleys or channels are providedalong which currents injected and blown in from the oppositely-arrangedburners can pass towards one another, these alleys or channels having atleast one upwardly opening aperture through which the gases ofcombustion can pass to the goods to be fired.

The manner of firing in accordance with the invention cannot be usedsimply by the application of pulsating burners. This is because with theordinary high rate of pulsation of the furnace atmosphere and, providingthat the fuel-air mixture injected has a high enough speed, annulareddies will result which do not move backwards and forwards across thekiln but which follow fixed paths leading to an unequal distribution oftemperature within the kiln. In other words the effect obtained with thepresent invention is different from that to be obtained merely withpulsating burners, with which, owing to the length of the flame changingin accordance with the pulsations, the only effect is the production ofa pulsating atmosphere around the burner. This pulsating kiln atmosphereeffects an improved mixture of the combustion gased with the combustionair and there is also a heating effect which varies in accordance withthe rhythm of the pulsations.

This stands in complete contrast to the principle underlying the presentinvention, because in accordance with the invention longer periods ofvariation in the flow conditions inside the firing space are createdwhich not only provide periodic changes in the flame action but whichalso bring about a substantial alteration in heat transmission byconvection. It is also particularly important that with the inventiontwo burners are arranged opposite one another and directed towards oneanother, so that the columns of gases produced by them interact directlyand cooperate to produce the desired periodic or cyclical variation inthe configuration of the heating currents within the kiln. In pulseburners, on the other hand, the same annular eddy current phenomena canappear as has been mentioned hereinabove as being disadvantageous, evenalthough such phenomena may only occur when the combustion airintroduced with the fuel has a sulficient injection speed.

Further, pulsating or pulse burners operate with a frequency of a fewcycles to substantially more than 1000 cycles per minute, whereas themethod in accordance with the invention involves a cycle whose lengthdepends upon the transverse dimensions of the kiln and is in the orderof magnitude of 30 seconds and more. Of course, with kiln cross-sectionsof small size the length of a cycle may be reduced very substantially.

The invention will now be further described with reference to theaccompanying drawings.

FIG. 1 shows a cross-section through a tunnel kiln with burners mountedin lateral recesses in the kiln walls.

FIG. 2 is a cross-section through a kiln Wall with a jet or beam-typeburner.

FIG. 3 is a cross-section through a tunnel kiln for producing an annulareddy using a beam-type burner.

FIG. 4 is a section through a tunnel kiln using two beam-type burnersdirected towards one another.

FIG. 5 diagrammatically shows the principle of the invention withreference to a cross-section through a tunnel kiln with two oppositelydirected burners.

FIGS. 6a, 6b, 6c, 6d and 6e diagrammatically show the course of eddymovement in a cross-section of a tunnel kiln using two burners directedtowards one another.

FIG. 7 shows a preferred manner of arranging articles to be fired inaccordance with the invention.

FIG. 8 is a section along the line A-B of FIG. 7.

FIG. 9 shows the horizontal surface on which goods to be fired aremounted.

FIG. 10 is a section on the line CD of FIG. 9.

Referring now to the drawings and more particularly to FIG. 1, it can beseen that the latter represents a crosssection through a tunnel kilnwhich is heated with normal burners which in the immediate proximity ofthe burner nozzles produce a restricted zone of heat development. Inthis figure reference numeral 1 is used to indicate a burner, referencenumeral 2 a lateral recess in which the burner is mounted, whilereference numerals 3 and 4 denote the side walls of the tunnel kiln inwhich the recesses 2 are formed. Reference numeral 5 denotes the roof ortop of the tunnel kiln. The kiln is provided with rails 6 extendingalong its length and carrying kiln trolleys 7 running on wheels 8 alongthe kiln. The trolleys carry stacks of goods to be fired, denoted byreference numeral 9. As illustrated, there normally is a relativelylarge distance between the burners 1 and the stacked goods 9, andbecause of this, temperature differences will normally occur in thefiring space 10 of the kiln, since there previously has been no way toobtain the necessary mixture of the high temperature burner gasesleaving the burners 1 with the kiln atmosphere.

In the case of the beam-type or jet burners shown in FIG. 2 the fuel,which in the example shown can be gaseous fuel, enters through the tube11 into the duct 12 along which it passes to the burner nozzle 13, towhich air for combustion is supplied via lateral duct means 14. The hotburner gases pass through a passage 15 and project outwardly into thecombustion space 17 in the form of an elongated jet or beam 16 which islimited by the wall 18.

The configuration of the eddies occurring Within the cross-section ofthe kiln with the use of such a burner can be seen from FIG. 3. Theburner is there denoted by reference numeral 21, while the line 22denotes the annular eddy which is produced in the kiln cavity 23 whensuch a burner is used. As can be seen from the drawing, in this annulareddy there is a dead or still space extending along the center of thekiln, so that consequently there is no even temperature distributionover the whole crosssection of the kiln.

Conditions can be improved somewhat if two burners 31 and 32 directedtowards one another are used, as shown in FIG. 4. In this case two flowsor streams 33 and 34 are formed, but there are still two practicallystill or dead spaces 35 and 36 within the kiln space 37, extending alongthe kiln.

FIG. 5 shows a modification of the kiln structure of FIG. 4, in whichthe effects of the present invention may be seen. The annular eddies 43and 44 normally produced by the burners 41 and 42 are shown in fulllines. On varying the quantity of the air-fuel mixture and/or the rateof heat development by the two oppositely placed burners 41 and 42, theannular eddies move across the kiln as is shown by reference numerals 43and 44'. This movement is shown in a sequence of views taken atsucceeding points in time in FIGS. 6a through 6e. FIG. 6a shows the eddymovement in accordance with the use of a burner as shown in FIG. 4, FIG.6b the eddy movement using two burners of which the left-hand burner 52receives less air fuel mixture or is caused to develop less heat thanthe right-hand burner 53. In the condition as shown in FIG. 60 thequantity of air fuel mixture supplied is the same as for both burners 52and 53 while in the case of FIG. 6d the air fuel mixture supplied or therate of supply of heat of the two burners 52 and 53 is exactly thereverse of that as shown in FIG. 6b. FIG. 66 shows further the extremecase in which only the lefthand burner 54 is in use, while theright-hand burner is out of operation. This is the reverse of operationwith the burner 51 as shown in FIG. 6a. Thus, in accordance with themethod of the invention the eddy currents can be caused to move acrossthe cross-section of the tunnel kiln in such a manner that thetemperature can be evened out in the kiln in a positive and reliablemanner and still or dead spaces extending along the kiln may becompletely eliminated.

The following table gives, by way of example, details of the relativeamounts of air-fuel mixture, expressed in arbitrary units, supplied tothe burners in the case of FIGS. 6a to 6e:

Left-hand burner:

If the kiln space is filled with material to be fired, there isnaturally a hinderance in the currents described hereinabove, becausethe material to be fired causes the currents to be diverted. Therefore,in many cases it is advantageous to stack the articles so as to leavechannels or alloys through the material to be fired in order to allowthe formation and circulation of eddies of burning gases in thechannels, of the type shown in the figures, wherein the products ofcombustion from the burners approach one another horizontally.

This can be carried out, for example, in the manner as shown in FIGS. 7and 8, wherein the firing of stacked rows of standard ceramic blocks orbricks is shown with alleys or channels left between the blocks andextending across the surface on which the blocks are mounted, at thesame level as the burners. The channels are formed by placing each rowon the top horizontal surface 71 of the kiln trolley so that the row isaligned with the direction of the alignment of the burners as disclosedabove, and so that the rows are parallel to each other. As depicted inFIG. 7, seach column of stacked articles is formed by two rows placedone on top of the other, the two rows vertically aligned so that they donot project across the channels 73. Thereafter, cross rows are placedcovering the channels formed by the first two rows.

It is also possible to leave spaces below the material to be fired, tothereby provide the space required for the circulating currents oreddies. This may be accomplished by using supports of the type known aselephants feet to those in the art, on the surface on which the kilncharge is to be arranged, The elephants feet can be so arranged thatalleys or channels result, as shown in FIGS. 9 and 10.

In FIGS. 9 and 10 reference numeral 81 denotes the top horizontalsupporting surface of the kiln trolleys for supporting the stack ofarticles.

In the embodiment shown in FIGS. 9 and 10 the elephants feet are shownat 82, and these are mounted on the stacking surface 81 standing alongthe kiln trolleys, with the articles to be fired being mounted atop theelephants feet 82. In this instance stacking is carried out on stackingplates or slabs 83 which, in accordance with a further feature of theinvention, are provided with corresponding openings 84 so that thecurrents of combustion gases can escape in an upward direction and runover the surface of the material to be fired.

It has been found especially advantageous, particu larly in the case oftunnel kilns, if combustion channels are provided on the firing trolleyswhich extend perpendicular to the longitudinal direction of the tunnelkiln, so that the burners discharge into them from both sides. Thesechannels conveniently have holes or apertures directed upwards and also,if required, in other directions, to allow for the combustion gases toescape in an upward direction and run over the surface of the materialto be fired. The channels can have side walls, perpendicular to thelongitudinal direction of the kiln, which are closed or provided withapertures.

In the case of chamber kilns or tunnel kilns as used in the ceramicsindustry, for example, the kiln is charged with the desired materialwhile heated by the burners located on opposite sides of the kiln. Inthis situation, it is generally believed that the supply of heat to thekiln must theoretically be held constant. In such a case it wouldtherefore be necessary to carry out the present invention by operatingthe burners in such a manner that the sum of the rate of heat output ofeach pair of oppositely placed burners directed towards one another isconstant at every instant. This means that the fall in heat output ofone burner per unit time must be made up by an equal increase in theheat output of the other bumer of the pair.

However, it has been found in practice that the actual amount of heatrequired in ceramic kilns does not in fact follow the theory regardingconstancy, but in reality appears to continuously vary. In order tocomply with this demand, the burners operated in accordance with theinvention must be so controlled that the sum of the amount of fuel andair mixture injected into the kiln and/ or the heat output of oppositelyplaced burners corresponds to the quantity of air-fuel mixture and/ orthe heat input required at every instant by the two burners.

The pulse burners which have come to be more and more widely used in theceramics industry and elsewhere can also be used in carrying out theprinciples of this invention. In these burners, liquid fuel isintermittently injected under high pressure into the firing space, andcombustion does not generally occur immediately at the outlet of theburner but instead occurs outwardly thereof a distance which may amountto several yards. In this case the amount of air required for combustioncan, for example, be supplied to every burner individually and injectedparallel to the fuel jet into the kiln space, with the principle of theinvention being applied by either varying the total quantity of injectedfuel and air, or by varying the heat output of the individual burners inaccordance with the rules described above.

The control of the burners themselves does not require any particulardescription, since known control elements can readily be applied. Thecontrol of the quantity of airfuel mixture injected can for example becarried out 'by means of chokes or with pressure-varying means in thesupply ducts. In the case of burners operated with liquid fuel thequantity of the air applied can also be varied by means of chokes or bymeans for varying the pressure, and the heat output can be varied byvarying the quantity of fuel supplied.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

1. In a method of heating a kiln having a kiln space with a roof, sidewalls and a floor, in which at least two burners are arranged at thewalls opposite one another and directed towards one another to propelproducts of combustion into'the kilnso as to cause circulatory heatcurrents therewithin, the improvement comprising cyclically changing thepositions of the paths of the heat currents produced by the burnersinside the kiln by cyclically altering the output of the burners soas'to cyclically move each such path first toward one such burner andthen toward the other thereof.

2. The method improvement of claim 1, wherein said burners arecontrolled by first increasing the output of one and decreasing theoutput of the other, and then increasing the output of said other anddecreasing the output of said one.

3. The method improvement of claim 1, wherein said burners arecontrolled by cyclical alterations in the airfuel mixture suppliedthereto.

4. The method improvement of claim 2, wherein the outputs of said oneand said other burner are simultaneously increased and decreased.

5. The method improvement of claim 3, wherein said burners arecontrolled by air-fuel mixture supplied thereto and of the heat outputproduced by the burners.

6. A method of heating a kiln having an elongated kiln space formed by aroof, side walls, and a fioor, comprising the steps: positioning atleast two burners in the said walls opposite one another and disposingsuch two burners towards one another so that they propel their productsof combustion into the kiln so as to produce circulatory currentsfollowing substantially loop-shaped paths flowing around quiescentcentral zones at a given instant, and changing the relative positions ofthe loopshaped paths of said circulatory currents by a cyclicalfluctuation of the current-propelling forces exerted by the burners, theburner on one side of the kiln producing a greater current-propellingforce While the other produces a lesser such force, and vice versa.

7. The method as set forth in claim 6, in which the frequency of thecyclical fluctuations of the two burners is varied.

8. The method as set forth in claim 6, in which the fluctuation cyclesof the two burners are of the same frequency and are about 180 degreesout of phase.

9. The method as set forth in claim 6, in which the instantaneoussummation of the amount of fuel supplied to the two burners remainssubstantially constant.

10. The method as set forth in claim 6, in which a plurality of suchpairs of oppositely-disposed burners is arranged in horizontalsuccession along the length of said kiln, each such pair undergoing saidcyclical fluctuation of the current-propelling forces of the burnerstherein, and in which a phase displacement of the fluctuation in thepropelling forces of each pair of adjacent burners is provided.

11. The method as set forth in claim 6, in which at least one passage isformed within the articles to be fired within said kiln, said passagebeing disposed such that the moving currents of the products ofcombustion from the two burners approach each other approximatelyhorizontally through such passage.

12. The method as set forth in claim 11, further including the step ofproducing from said horizontallyapproaching currents a heat currentwhich moves upwards from said passage through an upwardly-disposedopening in said articles.

13. A method of stacking articles for firing within a kiln of the typehaving horizontally disposed burners on opposite sides of said kilnaligned to direct heat currents at each other, the method comprising thesteps of providing a horizontal supporting surface to be moved throughthe kiln, placing a row of articles on said surface in alignment withthe direction of said alignment of said burners, leaving a horizontalheat approach channel on the two sides adjacent to said row, and placinga second row of articles on said surface parallel to said first rowadjacent to one of said channels.

14. The method as defined in claim 13, and further comprising the stepof placing at least an additional row of articles on top of both saidfirst and said second rows without projecting said additional row acrosseither of said channels.

References Cited UNITED STATES PATENTS 1,970,320 8/1934 Kier et al.26328 X 2,982,530 5/1961 Drakengren 26328 3,129,933 5/1964 Cremer et al26328 JOHN J, CAMBY, Primary Examiner US. Cl. X.R. 263-28

